Colorectal cancer (CRC) is the third leading cause of malignant death in the United States. Although environmental factors appear to be important in modulating CRC risk, the intracellular events associated with most disease-modifying exposure agents remain incompletely defined. Observations from a small number of retrospective studies suggest that some environmental factors, particularly those which might directly or indirectly influence the interrelated network of biochemical reactions involved in one-carbon metabolism, may be associated with alternate pathways of colorectal carcinogenesis. In the present application, we propose to examine the molecular epidemiology of sporadic CRC subtypes in a population-based cohort study. Our specific aims are to examine associations between cigarette smoking, estrogen exposure, and folate intake with distinct CRC subtypes defined by microsatellite instability phenotype, Ki-ras or p53 gene mutations, and CpG island methylator phenotype. The tissue resources generated for this project will also facilitate future, highly cost-efficient, analyses of other candidate exposures as new data emerge to support potential associations with specific genetic/epigenetic mechanisms of carcinogenesis. Thus, the PI (new investigator) should be able to derive extended benefits and demonstrate exceptional productivity from the current application. Achievement of the stated specific aims for our present study will capitalize upon data and tissue resources from the Iowa Women's Health Study cohort. Paraffin-embedded tissue samples will be collected from subjects who were diagnosed with incident CRCs between 1991and 2000. DNA will be extracted for genetic and epigenetic analyses and microscopy slides will be cut for immunohistochemical staining. To date, no prospective data have been reported regarding molecularly defined CRC risk associations. As designed, the proposed study addresses a research priority area identified by the NCI sponsored CRC Progress Review Group and is consistent with the objectives of a recent NCI Program Announcement (PA-04-099). Successful completion of this project should yield novel, informative data regarding the colorectal carcinogenic pathways influenced by common exposure agents. Categorization of sporadic CRCs into molecularly-defined subtypes is likely to be rewarding, because the increased tumor homogeneity should permit more accurate assessment of potentially etiologic risk associations. Further investigation of the molecular epidemiology of sporadic CRC should also permit the development of more precise pathogenic models for this disease, which may foster novel intervention strategies at multiple levels, including risk stratification, early detection, behavioral modification, chemoprevention, and/or chemotherapy.